The present invention relates to a method of producing a biaxially-stretched plastics material mesh structure, suitable for geotechnical and other civil engineering uses; a mesh structure employed for such uses can be referred to as a geogrid. The mesh structure is unbalanced, ie has a greater strength in a primary direction (PD) than in a secondary direction (SD) substantially at right angles to the PD. The method comprises providing a starting material which has a thickness (defined below) of at least about 2 mm and has a pattern of holes on a notional substantially square or rectangular grid whose axes are substantially parallel to the PD and to the SD respectively, the sides of PD end portions of at least some of said holes being defined by crotch-forming zones. The starting material is of square construction; ie the PD and the SD are aligned with the two orthogonal axes of the eventual product. PD stretch is applied to form oriented PD strands and to apply some orientation to the notional junction zones (defined below) so that orientation extends into and through the notional junction zones, from the end of one oriented PD strand to the adjacent end of the aligned oriented PD strand. SD stretch is applied to form oriented SD strands. At least part of the length of the edges of crotches connecting adjacent sides of adjacent PD and SD strands is oriented in the direction running round the crotch. The mid-point of the notional junction zone (defined below) in the mesh structure is significantly thicker than the mid-point of any oriented strand entering the notional junction zone.
The present invention also relates to a biaxially-molecularly-oriented integral plastics material mesh structure made from a starting material which has a thickness of at least about 2 mm and comprising oriented PD strands and oriented SD strands, interconnected by junctions with respective sides of adjacent oriented PD and oriented SD strands interconnected by crotches at least part of whose length is oriented with orientation running in the direction around the crotch. The orientation of the PD strands extends into and through the junctions. The junction mid-point thickness is significantly greater than the mid-point thickness of any oriented strand entering the junction.
The mesh structures of the invention were designed for applications in which the main tensile force will be applied in the PD, and the mesh structures have significantly greater strength in the PD than in the SD; for instance, when the mesh structure is attached to a vertical wall facing in a retained wall geoengineering construction, the main tensile force will be at right angles to the facing and can be aligned with the PD of the mesh structure.
U.S. Pat. Nos. 4,374,798 and 5,053,264 disclose mesh structures of the general type with which the present invention is concerned. U.S. Pat. Nos. 5,267,816 and 5,269,631 disclose mesh structures which have greater strength in the PD than in the SD.
From the point of view of PD strength and failure mode, the best structure is found to be a uniaxially-oriented (uniax) structure, as for example strengths of up to at least 160 kN/m width can be achieved with a 5.5:1 PD stretch ratio. However, in order to reduce the time taken to build a geotechnical construction whilst maintaining the strength of the construction, it is desirable to increase the width of the geogrid without reducing the PD strength per strand or the economy rating in the PD more than about 5% or 10%. As there is a limitation on the width of starting materials, this can only be achieved in manufacture by stretching in the transverse direction (TD). Thus although the improvements of U.S. Pat. Nos. 5,267,816 and 5,269,631 are primarily directed towards significantly increasing the machine direction (MD) strength by enabling one to apply greater PD stretches, the present invention is more concerned with being able to apply SD stretches without greatly reducing the PD strength per strand or the economy rating, whilst providing a satisfactory creep strain behaviour with adequate long term strength. Nevertheless, it is very difficult to produce biax products whose PD strength per strand or economy rating is similar to the PD strength per strand or economy rating of the intermediate uniax material if reasonable stretches are applied in the SD. This is because the SD stretch affects the PD strength of the junction. Also the SD strands must be sufficiently robust eg to reinforce soil.
It is desirable to have a low SD punch-out in order to leave as much material as possible in the PD strand-forming zones and produce a high strength product using not too thick a starting material. However, if the SD punch-out is decreased, the PD orientation through the junction becomes more difficult to control.
It is desirable that any alteration in the manufacturing technique should produce a mesh structure which appears robust to the eye (as this is important for sales), and that the mesh structure should have an acceptable failure mode. As a general principle, it is desirable to have failure in the PD strand and not in the junction as this enables better prediction of the long term performance of the product.
According to the method of the invention, crotch-forming zones in the starting material have protuberances. Each protuberance may be in the plan view shape of the hole, namely a portion of the crotch-forming zone which projects into the respective hole beyond a base line which is tangent to each end of the projecting portion. Alternatively or in addition, the protuberance may be out of the plane of the starting material, being a thickening when approached from the edge of the PD strand-forming zone and/or when approached from the edge of the SD strand-forming zone. In the special case where there is no thickening when approaching from the edge of the PD strand-forming zone (defined below) and also no protuberance as seen in the plan view shape of the hole, there is a tendency for the orientation to proceed too far around the crotch in the PD stretch. This tendency is avoided by ceasing PD stetching before the orienation has reached the thickening (which considered as one progresses from the PD strand-forming zone will be a decrease in thickness).
The size of said protuberance is such that during the stretch, at least part of the edge of the crotch interconnecting adjacent sides of adjacent oriented PD and oriented SD strands is oriented in the direction running around the crotch, but the orientation ratio decreases significantly as one passes around the crotch edge, either from the oriented PD strand or from the oriented SD strand. In the biax product, the crotch edge either a) has an unoriented part, or b) the thickness of the least oriented part of the crotch edge is reduced (or the length of the least oriented part of the crotch edge is increased) by no more than about 20% (or than about 15%, 10% or 5%, as alternatives) of its thickness (or length) prior to stretching. The action of stretching does not reduce the thickness of any point along notional ridge lines of maximum thickness on the product from the junction mid-point to the crotch edges to such an extent that the ratio of finished thickness to starting thickness at that point is less than about 80% (or than about 85%, 90%, 95%, as alternatives) of the ratio of finished thickness to starting thickness of the notional junction zone mid-point.
In the biaxially-stretched mesh structure of the invention, the junctions comprise a central zone which is thicker than thinner zones adjacent the ends of oriented PD and oriented SD strands. If the junction interconnects two oriented PD strands entering a junction from opposite sides and two oriented SD strands entering the junction from two other opposite sides, the junction central zone will be generally square or rectangular; the central zone need only be generally square or rectangular and its sides could be concave or convex. The central zone has a narrow pro jection at the corner extending outwards between said thinner zones, continuing through the crotch between the oriented PD strand and the oriented SD strand, and running into the crotch edge. Though the central zone is thicker than said thinner zones, the narrow projection may be thicker than the central zone. There is no point on a notional line of maximum thickness on the product from the junction mid-point to the crotch edge having a thickness of less than about 80% (or than about 85%, 90% or 95%, as alternatives) of the thickness of the junction. mid-point. The central part of the crotch edge zone can be convex. The mesh material defined above is the product produced from a starting material with flat or nearly flat faces; if a starting material face or faces diverge markedly from flatness, the product will be altered correspondingly.
The invention extends to a method of strengthening soil, comprising embedding in the soil the mesh structure of the invention or the mesh structure produced by the method of the inventio n, and to a geotechnical construction (a composite civil engineering construction) comprising a mass of soil strengthened by embedding therein the mesh structure of the invention or the mesh structure produced by the method of the invention.
The understanding of several of the following definitions is assisted by references in FIG. 1a of the accompanying drawings, but the definitions are not limited to the shape of hole 1 shown in FIG. 1a. 
The term xe2x80x9corientedxe2x80x9d means molecularly-oriented. In general, when an oriented strand is referred to, the preferred direction of orientation is longitudinal of the strand.
xe2x80x9cUniaxxe2x80x9d and xe2x80x9cbiaxxe2x80x9d mean uniaxially-oriented and biaxially-oriented, respectively. Substantially uniaxially-oriented means that on the surface of the structure, there has been extension of the mgterial in one direction but no substantial resultant extension of the material in the direction at right angles.
In relation to a mesh structure, xe2x80x9cbiaxially-orientedxe2x80x9d means that the mesh structure has been stretched in two directions generally at right angles to each other.
The xe2x80x9corientation ratioxe2x80x9d is the stretch ratio in a localised area.
The holes in the starting material may be through-holes or blind holes. If the holes are blind, the film or membrane in the hole will either rupture on stretching, or may remain as a thin membrane.
A xe2x80x9ctangent linexe2x80x9d is a notional line tangent to the ends of the holes on either side of a strand-forming zone, whether or not the hole is in accordance with the invention. FIG. 1a shows xe2x80x9cSD tangent linesxe2x80x9d 2 tangent to the PD ends of the holes 1 and xe2x80x9cPD tangent linesxe2x80x9d 3 tangent to the SD sides of the holes 1.
In extruded starting materials and in embossed or moulded starting materials, the holes are not normally vertical sided (ie perpendicular to the plane of the starting material). For extruded starting materials as a good approximation, the tangent line can be taken as the notional line tangent to the plan view see-through, ie the minimum hole size as viewed normal to the plane of the material, but ignoring film or feathered edges. For embossed or moulded starting materials, where the holes normally have sloping sides, as a good approximation, the tangent line can be taken as the notional line tangent to a point half way up the side of the hole, or, if the slope is different on each face, tangent to a point half-way between points half-way up the respective slopes, films or membranes in blind holes being ignored.
The xe2x80x9cnotional junction zonexe2x80x9d 4 is the zone of the starting material defined between the respective pairs of SD and PD tangent lines 2, 3. In the mesh structures, the notional junction zones are the zones of the surfaces of the structure which have been formed from the notional junction zones of the starting material.
When the plastics starting material is biaxially stretched in two directions substantially at right angles, the PD and SD, oriented PD strands and oriented SD strands are formed, extending in directions substantially at right angles to each other. The PD and SD strands are connected at junctions. At the junctions, the edge zone of each PD strand is connected to the edge zone of the respective SD strand by a continuous zone of plastics material which is located at the edge of the junction in the product and is formed from plastics material at the corner of the junction-forming zone in the starting material. This continuous zone of plastics material is termed the xe2x80x9ccrotchxe2x80x9d herein. The xe2x80x9cdirection running around the crotchxe2x80x9d as used herein and in the appended clause is the curved direction extending between adjacent oriented strands, either from the SD strand to the adjacent TD strand, or vice-versa.
A xe2x80x9ccrotch-forming zonexe2x80x9d 13 is the zone adjacent the corner of the respective notional junction zone 4, lying between a respective hole 1, the adjacent PD tangent line 3 and the adjacent SD tangent line 2.
A xe2x80x9cPD strand-forming zonexe2x80x9d 11 is the zone between respective holes 1 and at its ends between respective crotch-forming zones 13, which extends from the SD tangent line 2 tangent to the first PD ends of the respective holes 1 to the SD tangent line 2 tangent to the other, second PD ends of the holes 1.
An xe2x80x9cSD strand-forming zonexe2x80x9d 9 is the zone between respective holes 1 and between respective crotch-forming zones 13, which zone extends from the PD tangent line 3 tangent to the first SD ends of the respective holes 1 to the PD tangent line 3 tangent to the other, second ends of the holes 1. It is not necessary that orientation will pass along the whole of the SD strand-forming zone, and normally the very ends will not be stretched out.
A notional line of maximum thickness is a notional line taken from a point of maximum thickness on one edge of the structure to a point of maximum thickness on another edge of the structure and joining together points of maximum thickness in between.
xe2x80x9cStrictly uniplanarxe2x80x9d means that the material or structure is symmetrical about a median plane parallel to its faces. In general, a uniplanar starting material will give a uniplanar struture when stretched.
xe2x80x9cSubstantially uniplanarxe2x80x9d means that the material or structure does not deviate so much from strict uniplanarity that orientation is not comparable on each face of the biax product.
A xe2x80x9cstrictly flatxe2x80x9d starting material has monoplanar, parallel faces.
The xe2x80x9cstarting materialxe2x80x9d is the material immediately before initiation of the first stretch. However, when considering whether there is a thickening, the material should be as it would be without the application of the cold grooving technique described in U.S. Pat. No. 4,590,029, if such a technique has been applied, as the technique has little effect on the final orientation behaviour.
The xe2x80x9cpitchxe2x80x9d of holes is the distance apart of the centres of the holes in a specified direction.
xe2x80x9cPunch-outxe2x80x9d is the ratio of the maximum dimension of the holes (ie between the respective tangent lines) in a specified direction to the pitch of the holes in the same direction, whether or not the holes have been formed by punching or by another procedure which may not even involve material removal.
The terms xe2x80x9cthickxe2x80x9d and xe2x80x9cthinxe2x80x9d refer to the dimension normal to the plane of the material or mesh structure. Unless otherwise specified, the thickness is tne distance between the extreme faces at the thickest point. However, raised edges or tapered or feathered edges are ignored, as well as any minor grooves in the surface and any irrelevant projections from the surface.
The dimension xe2x80x9ctxe2x80x9d is the mean thickness of the starting material where the dimension a is measured, ie at the minimum SD dimension (width) or narrowest part of the PD strand-forming zone 11, the part that often determines the strength of the product. t is taken as the xe2x80x9cthickness of the starting materialxe2x80x9d, whether or not other parts of the starting material are thicker.
Where a crotch edge is referred to, eg for measuring the thickness of the crotch edge, a point is taken as close to the crotch edge (as seen in plan view) as possible, whether or not the edge is raised or somewhat tapered, though ignoring any feathering or membranous material that does not form part of the crotch proper, ignoring abrupt tapering required for mould withdrawal in moulded starting materials, and ignoring any radiussing caused by punching holes, though the reduction in thickness leading up to the radiussing is not ignored; it has been found that the crotch edge zone can be reduced in thickness by 9% to 10% as a result of punching alone.
The xe2x80x9cwidthxe2x80x9d is the dimension at right angles to the major axis of the zone in question, and xe2x80x9cnarrowxe2x80x9d relates to this dimension. xe2x80x9cNarrowxe2x80x9d means significantly longer than wide.
The stretch ratios are as measured after releasing the stretching force or after annealing if annealing is carried out, and as measured on the surface of the structure.
The xe2x80x9coverall stretch ratioxe2x80x9d is the stretch ratio applied to the whole length of the material.
xe2x80x9cHDPExe2x80x9d is high density polyethylene.
The xe2x80x9cstrengthxe2x80x9d of a mesh structure as used herein is the maximum strength per linear unit, measured in a normal tensile test, the linear units being in the direction at right angles to the direction in which the strength is measured, whether the strength is measured in the PD or in the-SD.
The xe2x80x9crib strengthxe2x80x9d is the maximum strength per strand.
The xe2x80x9ceconomy ratingxe2x80x9d is the PD strength of the product per unit width per mass per unit area, measured as kN per m per kg per m in the PD.
xe2x80x9cTruth linesxe2x80x9d are parallel lines applied (normally by printing or drawing) to the starting material, normally in two directions parallel to the PD and SD respectively.
The xe2x80x9cshoulder tangentxe2x80x9d 5 is the tangent to a side of the projecting portion or protuberance 6 at the point where the tangent makes the greatest angle with the PD, said angle being measured so that it is less than 90xc2x00 if the shoulder tangent 5 is directed towards the PD centre-line 7 of the hole 1 and towards the centre-line 8 of the adjacent SD strand-forming zone 9 and so that it is more than 90xc2x0 if the shoulder tangent 5 is directed towards the PD centre-line 7 of the hole 1 and away from the centre-line 8 of the adjacent SD strand-forming zone 9.
The xe2x80x9cshoulder anglexe2x80x9d is the angle xcex8 which the shoulder tangent 5 makes with the PD.
The xe2x80x9cneck tangentxe2x80x9d 10 is the tangent to a side of the protuberance 6 at the point where the tangent makes the smallest angle (positive or negative) with the PD, said angle being measured so that it is positive if the neck tangent 10 is directed towards the centre-line 8 of the adjacent SD strand-forming zone 9 and towards the PD centre-line 7 of the hole 1 and so that it is negative if the neck tangent 10 is directed towards the centre-line 8 of the adjacent SD strand-forming zone 9 but generally away from the PD centre-line 7 of the hole 1.
The xe2x80x9cneck anglexe2x80x9d is the angle "psgr" which the neck tangent 10 makes with the PD.
The dimension xe2x80x9caxe2x80x9d is the minimum SD distance between adjacent side-by-side holes 1.
The dimension xe2x80x9cbxe2x80x9d is the SD dimension between adjacent side-by-side holes 1, between the points where the respective neck tangents 10 are tangent to the sides of the protuberances 6 (measured at the point where the neck tangent 10 is coincident with the crotch edge and furthest from the PD end of the hole 1, if the respective part of the protuberance side is straight).
The dimension xe2x80x9ccxe2x80x9d corresponds to the dimension a, but is taken in the PD.
The dimension xe2x80x9cdxe2x80x9d corresponds to the dimension b, but is taken in the PD, between the points where the respective shoulder tangents 5 are tangent to the sides of the protuberances 6 (measured at the point where the shoulder tangent 5 is coincident with the crotch edge and furthest from the SD side of the hole 1, if the respective part of the protuberance side is straight).
The xe2x80x9cjunction diagonal ratioxe2x80x9d is the ratio of the diagonal dimension in the biax product across two diagonally opposed points of minimum or zero orientation of crotches and across a junction, to the same diagonal dimension i in the starting material.
The xe2x80x9cbase linexe2x80x9d 12 of the protuberance 6 is the line tangent to each end of the protuberance 6.
The xe2x80x9cprojecting extentxe2x80x9d j of the protuberance 6 is the greatest extent of the protuberance 6 as measured at right angles to the base line 12.
The term xe2x80x9csoilxe2x80x9d includes rocks, stones, gravel, sand, earth, clay, aggregate held by a binder such as asphalt, or any other particulate or cohesive material used in geotechnical engineering.
Using the invention, it has been found that the protuberance has made it possible to direct orientation positively through the junction, generally along the PD, while limiting the actual degree of orienation across the centre of the junction. The invention controls and limits the level of orientation in the crotches whilst nonetheless maintaining crotch edges which are oriented for at least part of their length in the direction running around the crotch. Although the crotches can be continuously oriented in the direction running around the crotch (ie xe2x80x9cuniaxialxe2x80x9d orientation), there are no crotches which are highly oriented from end to end.
Apart from the unoriented or low-oriented part of the crotch edge, there is a range of orientation ratio in the crotch edge extending continuously from the ratio in the unoriented or low-oriented part up to the orientation ratio of the respective oriented strand, or to a value even greater than that in the oriented strand. When PD stretch is applied, the protuberance provides a rapid increase in the cross-sectional area of the crotch-forming zone as the orientation progresses from the yield point(s) in a PD strand-forming zone towards the junction; the rapid increase is preferably followed by a region of more slowly increasing cross-sectional area. In other words the invention puts a block in the centre part of the crotch-forming zone, thereby preventing in a controllable manner high orientation passing all the way around the crotch edge. The invention also controls and limits any orientation passing behind the block, both in the PD stretch and in the SD stretch. The block is characterised in the product by a sharp increase in thickness (called a neck) in the crotch as one goes around the crotch from the side of the respective PD strand. For instance, one can obtain a very low crotch edge thickness reduction, of less than about 20% or even of less than about 5%, at the crotch-edge centre portion. As there will often be a sharp increase in thickness on either side of the unoriented or least oriented part of the crotch edge as this part is approached, the crotch edge thickness should be measured at its thickest point, or its extension should be measured at its part of least extension; thus, if it is difficult to measure the reduction in thickness of a crotch edge, its extension (ie orientation ratio) can be measured, for instance by drawing parallel lines across the median plane on the side face of the starting material (which parallel lines are as close together as practicable), and measuring their separation on the starting material and on the product (after biaxial stretching). The region having the increase in thickness can have a slope of about 45xc2x0 to the plane of the mesh structure, ie an included angle of about 90xc2x0.
The effect of the invention occurs even if the PD strand-forming zones form a large part of the SD width of the starting materal, and the invention can be used with low SD punch-out ratios; thus the SD punch-out can be less than about 60%, 55% or 50%, and preferably less than about 46%. This enables the economic manufacture of high PD strength products from relatively low thickness starting materials.
The invention eliminates or greatly reduces any tendency to splitting of transverse strands when the mesh structure is subject to PD forces. In other biax products, this can occur as a result of force transmission around highly oriented crotches to the adjacent SD strands; such splitting causes failure of the mesh structure in the junction zones and prevents the strength of the oriented PD strands being fully mobilised. In other words, in the structures of the invention, the junction stress pattern is more efficient when the structure is subjected to PD forces, and the structure can have a behaviour similar to that of a uniax structure. The PD and SD stretches do not interact or do not interact greatly, enabling the strength of a junction and hence the economy rating of a uniax structure to be maintained to a large extent in the final biax structure; thus it is preferred that during the SD stretch the mid-point of the junction does not reduce in thickness so that the orientation of the junction mid-point is uniaxial in the PD, though this is not essential. It is not necessary that all the SD strand-forming zone be stretched out on the SD stretch. The narrower very end portions of the hole (considered in the PD) can provide a narrow yield point for SD orientation and provide small bar widths (PD dimension) in a convenient manner; with suitable choice of PD pitches, the penetration of transverse orientation into the junction during the SD stretch is reduced whilst leaving enough material to form a satisfactory junction. There are no thin zones in the junctions such as could form tear starters and weaken the structure under the action of tensile forces.
An unexpected advantage of the invention is that the product can be very regular, even when produced using low (slow) strain-hardening materials, ie materials with a high natural draw ratio.
It has been found that a 4 m wide biax mesh structure which appears regular and robust to the eye can be formed from a starting material of about 1.5 m in width of a thickness not greater than 6.5 mm with a strength per meter width of 40 kN/m, having an acceptable failure mode and a satisfactory creep strain behaviour with adequate long term strength. Using the invention, it is possible to achieve resultant stretch ratios of up to 18:1 or more, for instance 5.5:1 in the PD and 3.3:1 in the SD. The PD stretch ratio (as measured from the mid-point of one notional junction zone to the mid-point of the adjacent junction zone in the PD) is preferably at least about 3:1 and more preferably at least 4.5:1 or 5:1. For higher strength products from a given starting material thickness, lower SD stretch ratios may be used, down to 1.5:1. The PD strength is significantly greater than the SD strength, eg at least about 1.5, 2, 2.5 or 3 times the SD strength. 6.5 mm sheet is sufficiently thin to be handled in production without particular difficulties though as machinery develops in the future it is anticipated that thicker materials, eg up to 15 mm, may be readily handled. In one example with about 8% crotch edge thickest part thickness reduction, the short term tensile behaviour of the biax product was found to be similar to that of the uniax intermediate product but the creep behaviour was found to be slightly worse than that of the uniax intermediate product.
As the orientation goes right through the junction in the PD stretch, the mid-point of the notional junction zone thins down, preferably by not more than about 10% and if possible by not more than about 5%. Due to pulling plastics material from other parts of the structure or due to relaxation of some of the orientation inserted during the PD stretching, the junction mid-point could increase in thickness during the SD stretch. Thus the final junction mid-point thickness is measured rather than the uniax thickness. The junction mid-point thickness will be significantly greater than the strand mid-point thicknesses, say at least twice the thickness, ie a difference in thickness which is not just due to random variation and which indicates that there is much greater orientation in the strand than across the junction. If desired, there can be continuous, substantially uniaxial orientation from end-to-end of the mesh structure, which provides reasonable long-term creep resistance, though this is not essential. In situations where long-term creep resistance is of less significance, one may use mesh structures of the present invention whose creep performance is inferior to that of a good custom-designed uniax mesh structure.
It is preferred that the junction diagonal ratio be close to unity, for example greater than about 0.8. If the ratio is unity, the diagonals of the junction have not altered in length during stretching, but usually the diagonals become shorter due to narrowing of the ends of the strand-forming zones and the junctions caused by stretching.
In general terms, the crotch-forming zone as seen in plan view as one progresses towards the PD end of the hole, can have:
i) a first part which widens out;
ii) a second part which does not widen out as rapidly as the first part;
iii) a third part which widens out more rapidly than the second part and terminates the crotch-forming zone.
Said first part is preferably immediately contiguous to the second part. Said second part is preferably immediately contiguous to said third part.
More specifically, the shape can be such that as one progresses towards the PD end of the hole:
iv) the side of said first part is progressively more inclined to the PD axis and is defined at least in part by a curve which is concave with respect to the hole;
v) the side of said second part is progressively less inclined to the PD (the angle of inclination can reduce to zero and then increase in the opposite sense) and is defined at least in part by a curve which is -convex with respect to the hole; and
vi) the side of said third part is progressively more inclined to the PD and is defined at least in part by a curve which is concave with respect to the hole.
The b:a ratio is preferably greater than about 1.5:1 or about 1.6:1, a specific preferred minimum value being about 1.64:1. The higher the b:a ratio, the greater the protection of the crotch and the less the orientation around the crotch. The maximum value of the b:a ratio is believed dictated by economical considerations (too much punch-out and lower strengths per meter width) rather than functional considerations and may be up to about 2.5:1 or up to about 3:1 or greater.
Consideration of the b:a ratio is relevant to starting materials where there are protuberances in the crotch-forming zones as seen in plan view and primarily where there is no change in thickness in the crotch-forming zone. However, the b:a ratio can be expressed as a ratio of the cross-sectional areas in the planes corresponding to those containing b and a. The cross-sectional area ratios are applicable to starting materials where there is a change in thickness or both a protuberance in plan view and a change in thickness. The planes would be those immediately before the change in thickness and immediately after the change in thickness. In general terms, one can consider the rate of increase in SD cross-sectional area over a certain PD distance.
At least up to 90xc2x0, the greater the shoulder angle xcex8, the greater the protection of the crotch and the less orientation around the crotch. Any inflection or protuberance enables a satisfactory product to be manufactured at reduced SD punch-out, and it is believed that the closer the protuberance is to having right-angled shoulders, the lower the SD punch-out that may be used if a certain mode of failure is preserved. The angle xcex8 is preferably more than about 50xc2x0, 60xc2x0, 65xc2x0, 70xc2x0 or 75xc2x0; the angle xcex8 is preferably less than about 135xc2x0, 125xc2x0, 115xc2x0, 110xc2x0 or 105xc2x0; the preferred angle xcex8 is about 90xc2x0.
The angle "khgr" between the shoulder tangent 5 and the neck tangent 10 is preferably roughly 90xc2x0 or an obtuse angle. The neck angle "psgr" is preferably about 0xc2x0. However, it is not excluded that the neck angle should have a negative value, ie with the hole 1 widening out again from a narrower xe2x80x9cneckxe2x80x9d as one approaches the PD end of the hole 1.
Considered in the SD, the crotch-forming zones can be as (i) to (iii) or (iv) to (vi) as set out above in relation to the crotch-forming zones as considered in the PD. The d:c ratio determines at least in part the amount the junction and crotch centre are thinned and oriented in the SD stretch. The d:c ratio is preferably greater than about 1.5:1 or greater than about 2:1; the ratio is preferably less than about 3:1 or less than about 4:1. By having the d:c ratio sufficiently large, any reduction of the thickness of the centre of the junction during the SD stretch can be avoided. In order to prevent the SD orientation penetrating too far towards the mid-point of the junction, the width c of the SD strand-forming zone 9 can be reduced or the bar thickness can be reduced, for instance by extruding material with PD grooves registering with the centre parts of the holes or for instance using the technique described in U.S. Pat. No. 4,590,029. The SD orientation can be taken beyond the line on which the dimension d is measured. It is preferred that during the SD stretch, the orientation does not extend so far around the crotch that the whole of the crotch is oriented.
The base line 12 of the protuberance 6 preferably makes an angle xcfx86 with the PD of mnore than about 30xc2x0 or 35xc2x0; the angle xcfx86 is preferably less than about 60xc2x0 or 55xc2x0. The projecting extent j can be about 3% to about 15%, preferably about 5% to about 10%, of the length of the diagonal i. If the base line 12 of the protuberance 6 is too short and if the projecting extent j is too great, there will be a tendency for the orientation to go behind the protuberance 6 as though the protuberance 6 were not there.
Preferably, the very LD ends of the holes 1 will be formed by continuous curves, though this is not necessarily so as the ends could be formed by straight SD lines. The sides of the crotch-forming zones 13 will be formed bycurves (which are not necessarily circular arcs) which can connect directly into one another or can be connected with one another by straight lines; the connections are smooth, ie no very small radius kinks or notches, at least where there is a connection that is concave to the hole 1. The radii should not be so small as to cause a notch and not so large as to impair the blocking function of the protuberance. The radii chosen can have a significant effect on the formation of the block.
Normally, it is preferred that each hole 1 be symmetrical about the PD axis and/or about the SD axis. The central parts of the sides of the PD strand-forming zones, ie the parts connecting said PD end portions, can be any suitable shape, eg of xe2x80x9cdiaboloxe2x80x9d shape as disclosed in U.S. Pat. No. 4,743,486, straight, or elliptical concave with respect to the hole 1. Grooving techniques such as the cold grooving described in U.S. Pat. No. 4,590,029 can be applied to the PD or SD strand-forming zones 11, 9.
The area of the PD strand-forming zone, as seen in SD section normal to the mesh structure between the respective PD tangent lines, is preferably less than about twice the area of the SD strand-forming zone as seen in section normal to the mesh structure and along the axis of the centre line of the SD strand-forming zone from the PD tangent line at one end of the zone to the PD tangent line at the other end of the zone. On a flat sheet, this ratio is equivalent to the SD punch-out.
Complex Starting Material Hole Configurations
The invention is applicable to more complex starting materials of square construction where some of the holes are different and are not on the same notional square or rectangle grid as the shaped holes of the inventions, eg as described in U.S. Pat. Nos. 4,536,429 or 4,574,100. However, in general, the invention is either applied to substantially every junction, or, with said more complex starting materials (where there will be junctions of different types in the biax products), the invention is applied to substantially every junction of a certain type.
Depending upon the configuration of the holes in the starting material, a plurality of junctions could be connected tog ether not by oriented strands but by unoriented or low-oriented material. In this manner, there can be a plurality of junctions connected in the PD and/or in the SD, the so-connected junctions interconnecting six or more oriented strands. The junction mid-point con tinues to be that point that corresponds to the mid-point of the notional junction zone, taking all holes into account unless they do not contribute to the formation of an oriented strand. Thus the invention is applicable to just one corner of a junction; each crotch which interconnects an oriented PD strand and an oriented SD strand can be of the form of the invention with the other crotches being of other forms. With such a mixed starting material, the dimensions a and b or c and d can be measured to the centre line of the respective PD or SD strand-forming zone.
In one form of starting material that provides such connected junctions, there are holes in accordance with the invention aligned in the PD; in the SD, holes in accordance with the invention are interspersed between other holes which are not in accordance with the invention, eg holes as disclosed in U.S. Pat. No. 4,743,486. In the SD, there can be one said other hole between hoies in accordance with the invention, or, if less SD extension is required, two, three or any suitable number of said other holes. The PD distance between said other holes may need to be greater than the PD distance between the holes in accordance with the invention. The holes are such that during the SD stretch, only the SD strand-forming zones between the holes in accordance with the invention stretch out. It is found that such a structure has better long-term creep performance, though its SD dimension will not be maximised, overall SD stretch ratios being reduced to say about 1.5:1 or less.
In another form of the invention, holes of the invention are interspersed in the PD between other holes which are not in accordance with the invention, the SD width of the latter other holes may need to be less than that of the holes. of the invention, ie in the PD, there may be wider holes (greater SD dimension) interspersed with narrower holes. The narrower holes between the holes of the invention can assist in producing the crotch block effect of the invention. On stretching in the PD, the narrow gaps between the PD ends of the wider and narrower holes prevent the crotch being stretched right out and assist in the formation of a block; on stretching in the SD, the narrow gaps act as narrow yield points and assist in preventing large forces being applied across the junction.
In another form of starting material, the invention is applied to one side of the hole but not to the other side of the hole, giving a hole which is asymmetrical about its PD axis.
There may be other ways of achieving the crotch block effect.
There is available a mesh structure in which the SD oriented strands are each wholly or partially divided into two generally superimposed strands which may be somewhat displaced relative to each other in the PD, looking at the structure in plan, formed from an extruded starting material in which a slot is formed in the PD through the middle of the SD strand-forming zone. The present invention could be applied to such a starting material. Structures produced in this manner are expected to have the advantages of the invention.
Starting Materials
The starting materials can be in general as set out from column 10, lines 20 to 24 and from column 10, line 32 to column 11, line 6 of U.S. Pat. No. 5,269,631, which disclosure is incorporated herein by reference. The preferred plastics materials are polyolefins. The experimental work on which the present invention is based was performed on medium molecular weight HDPE, which has a relatively low rate of work hardening. Other material, and also factors such as different stretching temperatures and rates and different degrees of restraint at right angles to the stretching direction, can significantly change the mesh structure produced. Thus, test pieces of the starting material should be stretched on a laboratory scale to determine whether they perform in accordance with the invention.
The holes 1 can be made in any suitable way; the preferred way is to punch, though moulding is possible and in theory a Hureau-type or Rical-type or Sire-type die head (as in FR 2 131 842 or U.S. Pat. No. 3,252,181) could be used if the hole shape and thickness profiles can be sufficiently well defined. The starting materials need not be strictly or even substantially flat and can be for instance as disclosed in U.S. Pat. No. 5,053,264 or the bumpy materials which may be formed by extrusion with the Hureau-type or Rical-type or Sire-type die head. Normally, a moulding process and a Hureau-type or Rical-type or Sire-type die head provide holes with tapered sides. The starting materials need not be substantially uniplanar though are preferably so. The starting materials may be grooved as described above or may be specifically profiled, eg as described in U.S. Pat. No. 5,267,816. The starting materials are preferably not substantially oriented, though melt flow orientation can be present.
Testing
As the behaviour of the starting material varies with many factors such as the resin used, the thickness, the stretching temperature, the hole shapes and the hole pitches, laboratory samples should be produced and tested before carrying out tests on a production line, to ascertain whether the desired orientation behaviour is achieved. The application of truth lines to a laboratory sample greatly assists in observing the orientation behaviour.
Production Plant
Though not essential, the PD stretch will normally precede the SD stretch. In production, the PD will often be the machine direction (MD), but it could be the transverse direction (TD). Although it is normally more economical to carry out sequential MD/TD stretching with the TD stretch following the MD stretch, it is not essential for the present invention that the TD stretch should follow the MD stretch. Thus the PD stretch can be carried out before or after the SD stretch. Furthermore, multi-stage stetching can be employed, particularly in the PD; in such a case, the PD orientation need not be taken right through the junction until the final PD stretch. In normal manufacturing practice, the mesh structure is formed as a long MD length which is rolled up. It is not necessary to cool between stretches, and all or any two or any three stretching operations can be carried out in-line without cooling.
The plant illustrated in FIG. 11 or 11a of U.S. Pat. No. 4,374,798 can be used in commercial production to carry out the stretching operations of the inventions in two stages; alternatively the stretching may be in three or more stages, eg as described in U.S. Pat. No. 5,269,631.
Uses
The principal use is for strengthening soil and making a geotechnical structure, as referred to above, when the biaxially-stretched structure would be referred to as a geogrid. The uses set out from column 17, lines 45 to 61 and from column 17, line 64 to column 18, line 59 of U.S. Pat. No. 5,269,631 are appropriate for the mesh structures of the present invention. The mesh structures of the present invention are particularly suitable for retaining walls or embankments, with the PD generally at right angles to the face of the wall or embankment as seen from above. However, the mesh structures can be used for other purposes. For instance, the mesh structures can be used in asphalt road constructions, particularly to avoid reflective cracking, or can be used in concrete constructions in order to control shrinkage cracking.